1. Field of the Invention
The present invention relates to a developing agent to be employed for the development of an electrostatic latent image in electrophotography, etc. In particular, the present invention relates to a developing agent formed of a combination of full color toners comprising color toners such as a yellow toner, a magenta toner and a cyan toner, and a black toner.
2. Description of the Related Art
In recent years, as an image output apparatus based on electrophotography such as a copy machine, a printer, etc., there have been developed, in addition to a conventional monochromic image output device using only a black toner, a full color copy machine or full color printer which makes it possible to reproduce a wide range of colors through the employment of three primary colors such as yellow, magenta and cyan and further through a suitable superimposition of these color toners with a black toner, and these full color copy machines and printers are now available on the market.
In order to realize excellent color development and excellent reproduction of color image, the full color image to be obtained through such a full color apparatus should be such that the image portions thereof be formed from a color toner, in particular, through the superimposition of color toners of two or more kinds are required to be rendered in a state where the particles of the color toners are sufficiently fused and mixed in color, and furthermore, the image portions are required to have a suitable degree of glossiness so as to give a feeling of high-class and high quality to the full color image. In particular, if a full color image is to be formed on the surface of an OHT sheet, the image portions are required to be smooth and excellent in glossiness in order to realize excellent color development of projected image, thereby suppressing the scattering and irregular reflection of light that may be caused due to the irregularity of the surface of image portions, thus ensuring a sufficient degree of light transmittance at the image portions.
On the other hand, in view of the needs demanded by the users working in ordinary offices, in particular, it is increasingly needed to provide a full color copy machine and a full color printer which are capable, in addition to the situation where a full color image is to be produced, of creating a situation where only black toner is employed to produce a monochromatic image without undergoing the development of full color toners, to thereby make it possible to realize such a high speed image-producing capability as obtainable in the ordinary monochromatic copy machine or monochromatic printer. Further, with respect to the monochromatic image to be obtained, the qualities and features which greatly differ from those of a full color image are being pursued. Namely, since the monochromatic image is mainly intended to depict the image of a letter or character of a document, the glossiness of the image portions is required to be suppressed in order to minimize the reflection of light to be generated therefrom, thus alleviating the burden on one's eyes to thereby allow the letter and character to be easily identified.
As a matter of fact, however, the full color copy machine and the full color printer according to the prior art are incapable of meeting the aforementioned requirements when they are subjected to the monochromatic image-forming output. One of the reasons for this resides in the fact that the black toner to be employed in the conventional full color copy machine and full color printer is generally made so as to have almost the same degree of viscoelasticity as that of color toners, so that the image formed by the black toner is as high in glossiness as that formed by color toners, thus inevitably resulting in the formation of a monochromatic image, which is not suited for identification where the monochromatic image is a document or letter, etc.
There is another problem which will be attributed to the fact that, as the number of monochromatic images output is increased, the conventional full color machine is apparently disadvantageous in terms of durability in view of the construction of the fixing device mentioned above, i.e. the conventional full color machine is far inferior in durability to the ordinary monochromatic copy machine or monochromatic printer.